This invention relates to the field of spinal surgery. More specifically, this invention relates to a novel implantable apparatus for replacing the functionality of one or more failing intervertebral discs, without fusing the vertebral bodies above and below the disc(s). This invention also relates to devices for implanting and securing the intervertebral prosthetic device in cavities in a vertebral body and in one or more adjacent intervertebral discs. The invention further relates to methods for performing spinal surgery.
The human spine is a flexible structure comprised of twenty-three mobile vertebrae. Intervertebral discs separate and cushion adjacent vertebrae. The top and bottom surfaces of intervertebral discs abut vertebral body endplates. The intervertebral discs act as shock absorbers and allow bending between the vertebrae.
An intervertebral disc comprises two major components: the nucleus pulposus and the annulus fibrosis. The nucleus pulposus is centrally located in the disc and occupies 25-40% of the disc's total cross-sectional area. The nucleus pulposus usually contains 70-90% water by weight and mechanically functions like an incompressible hydrostatic material. The annulus fibrosis surrounds the nucleus pulposus and resists torsional and bending forces applied to the disc. Thus, the annulus fibrosis serves as the disc's main stabilizing structure. A healthy disc relies on the unique relationship of the nucleus and annulus to one another.
Individuals with damaged or degenerated discs often experience significant pain. The pain results, in part, from instability in the intervertebral joint due to a loss of hydrostatic pressure in the nucleus pulposus, which leads to a loss of disc height and altered loading of the annulus fibrosis.
A conventional treatment for degenerative disc disease is spinal fusion. In one such surgical procedure, a surgeon removes the damaged natural disc and then fuses the two adjacent vertebral bodies into one piece. The surgeon fuses the vertebral bodies by grafting bone between the adjacent vertebrae and sometimes using metal rods, cages, or screws to hold the graft in place until the graft heals.
Although spinal fusion may alleviate pain associated with degenerative disc disease, it also results in loss of motion at the fused vertebral joint. Lack of motion at the fused site puts additional stress on the discs above and below the fusion. The additional stress may cause the adjacent discs to degenerate and produce pain, thereby recreating the original problem.
To remedy the problems associated with spinal fusion, various prosthetic devices have been developed either to replace the entire disc (i.e., the nucleus pulposus and the annulus fibrosis) with a prosthetic joint or to replace the nucleus pulposus of the damaged disc with a suitable biomechanical equivalent. Unfortunately, the previous approaches have certain limitations because conventional total disc replacement devices and nucleus replacement devices disrupt tissues that will not heal.
In the case of total disc replacement surgery, existing prosthetic devices have met with limited success in reproducing the biomechanics of a natural disc. Moreover, the anterior longitudinal ligament must be severed as part of the anterior approach by which the device is implanted. Worse, the severing may span two vertebral bodies for a two level reconstruction, which can lead to lessened spinal function and stability. Further, total disc replacement devices require removal of a substantial portion the disc and attachment to the adjacent vertebral bodies. The endplates of the vertebral bodies are nonuniform and typically sclerotic, which prevents the close physical joining of endplate and device surfaces required for bone ingrowth to provide adhesion and can lead to subsidence of the disc replacement device into the bone of the vertebral bodies if the endplates are shaved for contour matching. Moreover, the devices display limited motion. Specifically, as a result of the oversized implant relative to the narrow disc space, total disc replacement often results in a range of motion of only about 3.8° to 4.6°. Such a limited range of motion is the equivalent of a spinal fusion, which is defined to be motion of less than about 5°.
For example, U.S. Pat. No. 4,759,769 to Hedman et al. discloses a synthetic disc having upper and lower plates hinged together. Although the hinged disc allows forward bending between adjacent vertebrae, the hinged disc does not allow axial compression or lateral flexion. Nor does it allow axial rotation of the vertebral column at the site of the implant. Therefore, the Hedman et al. device lacks the biomechanics of a natural disc.
Likewise, the prosthetic disc device disclosed in U.S. Pat. No. 4,309,777 to Patil does not replicate natural motion between adjacent discs. The Patil device includes two cups, one of which overlaps the other and is spaced from the other by springs. The cups move only in a single axial dimension. Thus, the Patil device does not enable natural flexion of the spine in any direction. In addition, the highly constrained motion of the Patil device can lead to high device/tissue interface stresses and implant loosening.
In the case of nucleus replacement devices, historically these devices required perforation or partial excision of the annulus to insert the device. Breaking the continuity of the annular ring precludes normal stress loading of the annulus, which may be necessary for later healing. Further, degeneration of the annulus, exacerbated by damage done during implantation, may also result in increased loads placed upon the implant. Increased loads of this nature may lead to subsidence of the device into the vertebral body, device extrusion through the annular defect, or expulsion from the nuclear space. Moreover, these problems are exacerbated in the situation in which more than one disc is to be replaced because any or all of the devices may develop these problems. These problems are particularly challenging in the lumbar spine, where the discs are most highly stressed due to high bearing requirements.
A remarkable intervertebral synthetic prosthetic device that greatly reduces the problems associated with total disc replacement and conventional nucleus replacement devices is disclosed in U.S. Pat. No. 5,827,328 (“the '328 patent”) to Buttermann. The Buttermann devices excise the nucleus pulposus while maintaining the biomechanical functionality of the intact annulus fibrosis. Moreover, the intervertebral prosthetic device permits at least four degrees of relative motion between two vertebral bodies on either side of targeted intervertebral disc. These degrees of relative motion include sagittal bending, coronal bending, axial rotation, and axial compression. Moreover, the compressible member permits small increments of translational movement between the vertebral bodies (i.e., fifth and sixth degrees of relative motion, namely anterior-posterior translation and side-to-side, or lateral, translation).
FIG. 1 shows an embodiment of an intervertebral prosthetic device 10 according to one embodiment of the '328 patent that is designed to replace a damaged natural disc. This device 10 is implanted by making holes in two adjacent vertebral bodies and boring through the nucleus pulposus of the intervertebral disc between the vertebral bodies. The intervertebral prosthetic device 10 has a first fixation member 14, a second fixation member 16, and a compressible member 18 that is positioned between the first fixation member 14 and the second fixation member 16. In addition to restoring the disc height, the compressible member 18 acts as a shock absorber to minimize impact loading and, thus, minimize device failure or vertebral fracture.
The first fixation member 14 is positioned in a first vertebral body 20. The second fixation member 16 is positioned within a second vertebral body 22 adjacent the first vertebral body 20. Each fixation member 14, 16 has an adjustable member 28, 30, respectively, and a support member 32, 34, respectively. Controlling the height of the adjustable members 28 and 30, along with selecting an appropriately sized support member, controls the “disc” height. The disc height is defined as the axial distance between the vertebrae above and below the operative disc.
The adjustable member 28 of the first fixation member 14 has an imaginary first longitudinal axis (shown by double-arrowed line A-A in FIG. 1) and adjustment elements 24 that allow adjustment of the height of the adjustable member 28 substantially along its longitudinal axis. In the embodiment shown in FIG. 1, the second fixation member 16 is structurally similar to the first fixation member 14, but inverted. The adjustable member 30 of the second fixation member 16 has a second longitudinal axis (shown by double-arrowed line B-B) and adjustment elements 26 that allow adjustment of the height of the adjustable member 30 substantially along its longitudinal axis.
FIG. 4 shows one embodiment of the first fixation member 14. In the embodiment shown in FIG. 1, the second fixation member 16 is structurally similar to the first fixation member 14, but inverted. Thus, the following discussion also applies to the second fixation member 16.
The adjustable member 28 of the first fixation member 14 is adjustable in an axial direction by adjustment elements 24. The adjustment elements 24 comprise telescopic struts extending between a first, outer plate 31 and a second, inner plate 33. The outer plate 31 is farther from the operative intervertebral disc and hence farther from the compressible member 18. In contrast, the inner plate 33 is closer to the operative intervertebral disc area and hence closer to the compressible member 18. In the embodiment illustrated in FIG. 1, the outer plate 31 has a bone-contacting surface 27, and the inner plate 33 has a surface 35 for positioning against the support member 32.
The adjustment elements 24 adjust the distance between the first bone-contacting plate 31 and the second plate 33, thus adjusting the height of the adjustable member 28. A surgeon may adjust the telescopic struts to increase the height of the adjustable member and thus mechanically pre-load the compressible member 18 to reproduce the axial compression absorbed by a nucleus pulposus of a natural disc. Pre-loading the compressible member restores the intervertebral height at the operative joint, restores the function of the annulus fibrosis. Pre-loading also assures close apposition of an ingrowth surface 27, 29 of the device to bone 36, 38.
Each telescopic strut is provided with a lock screw 63 to adjust the length of the strut 24 and hence control the height of the adjustable member. The lock screw 63 may comprise, for example, a pin (not shown) that extends through both the telescoping portion 65 and the housing portion 67 of the strut 24. Each strut 24 is independently adjustable. FIG. 5 shows a top view of the second plate 33 of the adjustable member 28. The adjustment elements 24 preferably are spaced equidistant from each other to enable specific height adjustment of various regions of the adjustable member.
The first and second fixation members 14 and 16 have porous portions, such as the bone-contacting surface 27, to permit bone ingrowth. In FIG. 1, the bone-contacting surface 27 of the adjustable member 28 is positioned against the subchondral bone of an endplate 36 of the superior vertebral body 20, and the bone-contacting surface 29 of the adjustable member 30 is positioned against the subchondral bone of an endplate 38 of the inferior vertebral body 22. Alternatively, a biocompatible fabric or suitable material may be wrapped around the fixation members to enable bone ingrowth. As another alternative, a biocompatible coating may be applied to the fixation members to facilitate bone ingrowth. The prosthetic device of FIG. 1 does not require conventional mechanical attachments, such as pegs or screws, to hold the prosthesis permanently in place. The intravertebral (i.e., within a vertebral body) positioning of the fixation members 14, 16 maintains the prosthetic device 10 in stable relationship at the operative intervertebral joint. The prosthetic device, however, may include mechanical or other attachments to supplement the porous portions of the fixation members and to temporarily fix the prosthetic device in place until bone ingrowth has occurred.
To further promote bone ingrowth, the adjustment elements 24 may include fins 66 extending outward from an exterior surface of the element 24, as shown in FIG. 4. The fins 66 increase the surface area of the fixation member 14 to which bone may attach. Preferably, these fins 66 are located on the adjustment elements that are positioned on the anterior side of the adjustable member 28. The prosthetic device also may include protuberances (not shown) on the bone-contacting surface of the adjustable members to increase the surface area of the porous portion of the fixation members and, thus, encourage bone ingrowth.
FIG. 6 shows a cross-section of support member 32. The support member 32 has a first surface 72 that operably faces away from the compressible member 18 and a second surface 74 that operably faces towards the compressible member 18. The first and second surfaces 72 and 74 are oblique so that a circumferential surface 77 around the support member 32 varies in width, as shown in FIG. 4. Thus, the support member 32 is wedge-shaped. In other words, the support member 32 preferably tapers from a maximum thickness at one side 73 to a minimum thickness at an opposite side 75. Generally, the support member 32 is thicker on the side of the fixation member 14 placed anteriorly in a patient's spine to account for the spine's natural curvature.
The support members are constructed with various thicknesses and with various angled surfaces, depending upon the vertebral level of the operative intervertebral joint. An angle θ shown in FIG. 6 ranges between 3°-10°. The support members are shaped to maintain sagittal alignment. Maintaining sagittal alignment avoids nonuniform loading of the compressible member and avoids early fatigue failure of that member.
The compressible member 18, which is shown in FIG. 2, can comprise at least one spring and, in the illustrated embodiment, comprises a plurality of springs 40. The compressible member 18, which is implanted in the region of an excavated nucleus pulposus of the operative intervertebral disc, is dimensioned so that the annulus fibrosis of the natural disc is at least substantially (if not completely) maintained. As a result, the intervertebral prosthetic device restores the mechanical properties of the disc without disrupting the annulus fibrosis. Retention of the annulus fibrosis maintains stability of the intervertebral joint at the implant site. In addition, the annulus fibrosis serves as a boundary for the compressible member and, therefore, minimizes the potential for accidental dislodgment of the prosthetic device.
The compressible member 18 has a top plate 42, a bottom plate 44, and a plurality of coil springs 40 extending between the top plate 42 and the bottom plate 44. The top plate 42 has a first surface 46, which is connectable to the first fixation member 14, and a second surface 48. The bottom plate 44 also has a first surface 50, which is connectable to the second fixation member 16, and a second surface 52. The springs 40 extend between the second surfaces 48 and 52 of the top plate 42 and bottom plate 44, respectively.
When pre-loaded, the compressible member 18 can have an axial height of approximately 1.5 cm, greatest at the L45 vertebral level and slightly less at the upper lumbar vertebrae. The coil springs 40 can have non-linear stiffness so that they become stiffer at higher applied loads; the nonlinear stiffness simulates physiological intervertebral stiffness. Moreover, any spring arrangement may be used that achieves sufficient axial compressive stiffness to replicate the biomechanics of the natural disc.
The compressible member includes an imaginary longitudinal axis (shown by the dashed line C-C) and an outer periphery in a plane transverse to its longitudinal axis. A largest dimension of the compressible member's outer periphery is less than or substantially equal to the diameter of a nucleus pulposus of the natural intervertebral disc. Put another way, the annulus fibrosis of the natural disc, which is substantially (if not completely) preserved during the implantation procedure, circumscribes the compressible member 18. For example, where the compressible member comprises a plurality of springs, the outer periphery of the compressible member circumscribes the springs, and the largest dimension of that outer periphery may extend to, but does not extend beyond, the nucleus pulposus. In other embodiments, where the compressible member includes a top plate and a bottom plate, and where those plates fit within the annulus fibrosis and extend beyond the outermost portions of the springs, the outer periphery of the compressible member equals the larger of the two plate peripheries. In quantitative terms, the outer periphery of the compressible member preferably ranges between 2.0 cm to 3.0 cm, which approximates the diameter of the nucleus pulposus of a natural intervertebral disc.
FIGS. 3A-3C show three embodiments of a coil spring designed to possess non-linear stiffness. In the embodiment of FIG. 3A, the coil spring 54 has a variable, or non-uniform, cross-sectional diameter 56. FIG. 3B shows another embodiment in which a coil spring 58 has a variable pitch 60, where the pitch is defined as the distance between successive coils of the spring 58. FIG. 3C shows a third embodiment of a coil spring 62 in which at least two of the spring coils have different radii 64 measured from an imaginary axis D-D extending along the central axis of the spring 62.
A method of intervertebral disc replacement now will be described in connection with FIGS. 8-14. FIG. 8 shows a pathological intervertebral disc 90 located between a superior vertebral body 92 and an inferior vertebral body 94. Prior to implantation, a surgeon performs a partial vertebrectomy to excise bone matter from the superior vertebral body 92 for receipt of a fixation member. This procedure can be performed using a cutting guide and reamer. Bone harvested from the vertebral body 92 by the reamer can be used after implantation of the prosthetic device to promote bone ingrowth into the prosthetic device, as later described. The partial vertebrectomy creates a cavity bounded by subchondral bone of a distant endplate 96 and subchondral bone of a near endplate 98 of the superior vertebral body 92. FIG. 9 shows a cross-sectional view of the superior vertebral body 92 after the partial vertebrectomy, as taken along line 9-9 in FIG. 8.
The surgeon next excises the nucleus pulposus of the damaged disc to create a cavity 100, as shown in FIG. 10, for receipt of the compressible member. The annulus fibrosis 102, seen in FIG. 11, is maintained.
Upon completion of the partial vertebrectomies, the surgeon implants a fixation member 104 into the inferior vertebral body 94, as shown in FIG. 11. The surgeon can select a support member with an appropriate thickness to accommodate the angulation at the operative intervertebral levels. The surgeon then inserts a compressible member 106 (via the cavity formed in the superior vertebral body 92) into the cavity formerly containing the nucleus pulposus of the damaged disc and connects it to the inferior fixation member 104, as shown in FIG. 12. The compressible member 106 and the fixation member 104 may be connected by conventional attachment members, such as screws, or by biocompatible cement or a suitable adhesive composition. Finally, the surgeon implants another fixation member, similar to the one implanted in the inferior vertebral body 94, yet inverted, in the superior vertebral body 92. Connection of that fixation member to the compressible member 106 forms an intervertebral prosthetic device like the one shown in FIG. 1.
Once the fixation members are in place, the surgeon expands each adjustable member. The surgeon applies distraction until the adjustable member is seated against the subchondral bone and distant endplate 96 of the vertebral body and until the desired compression has been applied to the compressible member. The adjustment elements of the adjustable member are then secured, e.g., FIG. 13 shows rotation of the lock screws 112 of individual telescopic struts 108 to secure the struts at an appropriate height.
The surgeon next packs cancellous bone grafts 118, typically obtained during creation of the cavity in the vertebral body, around the struts of each adjustable member, as shown in FIG. 14. The growth of bone around the fixation member and into its porous surfaces secures the intervertebral prosthetic device in place, absent mechanical attachments typically used in conventional disc prostheses. The surgeon then replaces the cortical bone from the partial vertebrectomy procedure and, if needed, secures it with a bone screw, suture or bone cement. In certain clinical situations, as when there is poor bone healing or insufficient bone, the surgeon may elect to use bone cement to attach the fixation members to the vertebrae.
Although the embodiment shown in FIGS. 1-6 is effective, in some instances it may be unnecessarily invasive as a result of its implantation via two vertebral body holes. FIG. 7 shows a second, less invasive embodiment described in the '328 patent, in which a prosthetic device is implanted via one vertebral body hole.
FIG. 7 shows an intervertebral prosthetic device 76 according to this second embodiment that comprises a first fixation member 78, a second fixation member 80, and a compressible member 82. The compressible member 82 is positioned between the first and second fixation members 78, 80. The second fixation member 80 comprises a wedge-shaped support member with an upper surface 84 that attaches to the compressible member 82 and a lower surface 86 that rests upon subchondral bone of a near endplate 88 of an inferior vertebral body. In this embodiment, adjustment of the first fixation member 78 pre-loads the compressible member 82 to an appropriate extent. Also, in this embodiment, a lower surface 86 of the support member 80 may be composed of a porous material and may have a slightly convex shape to match the natural contour of the near endplate of the inferior vertebral body.
The implantation of the FIG. 7 embodiment is similar to the implantation of the FIG. 1 embodiment. Specifically, similar to the embodiment shown in FIG. 10, a cavity may be formed in the superior vertebral body 92 and then extended through the nucleus pulposus of the intervertebral disc therebelow. At this time, the compressible member with the lower fixation member 80 affixed thereto may be inserted through the cavity in the vertebral body and then pushed downward into the cavity 100 in the intervertebral disc. Subsequently, the upper fixation member 78 is: (a) positioned in the cavity formed in the superior vertebral body 92; (b) connected to the compressible member; and (c) adjusted in the manner previous discussed with respect to the FIG. 1 embodiment. Of course, the cavity in the superior vertebral body 92 is then closed also in the manner previously described.
As evident from the embodiments of FIGS. 1 and 7, the intervertebral prosthetic device embodiments have a modular design so that the prosthesis may be sized to the patient's anatomy and designed for the patient's condition. The modular design also enables replacement of individual components of the prosthesis (i.e., a fixation member or a compressible member), rather than replacement of the entire prosthesis should one component fail. As a result, the compressible member can be attached to the fixation members by mechanical attachments, such as screws, rather than bone cement so that a surgeon may easily replace damaged or worn components.
Unfortunately, the embodiment shown in FIG. 1 precludes use when reconstructing multiple adjacent discs. Additionally, although the less invasive embodiment shown in FIG. 7 may be implanted via only one vertebral body hole, it may be less effective than the embodiment shown in FIG. 1 when used in patients with low bone density. Specifically, the FIG. 7 may be less effective as a result of inability to adequately fix the lower fixation member 80 to the vertebral body below the compressible member. Further, this inability to adequate fix the lower fixation member 80 may, in turn, lead to subsidence of the device into the vertebral body adjacent the lower fixation member 80.